Possible Proxy server issues

Noob middleware guys here. We have a websphere app that works fine externally, but from within the office it consistantly gets 'page cannot be displayed' errors. We suspect the app and proxy server are not playing well together. A network guy traced all machines involved for me as I recreated the issue. I've opened them in Wireshark ... and am stuck. Can anyone suggest the best way to filter, pick out errors, anything I should try first to start troubleshooting. Using Fiddler, I did find some 407 and 502 (proxy server related) errors. Thanks !